Invulnerable is an unused but still functional creature spell in Dungeon Keeper 2. As the name implies, it causes its target to become invulnerable for a brief moment, making them able to avoid damage.
The spell itself is not available in-game, and has to be added to a creature using the Dungeon Keeper 2 Editor. Once cast, a magical blue cloud is launched forward, and if it hits a friendly target, causes them to be circled by similarly coloured blue shapes, and allowing the target to avoid damage for a short while.
The effects of this spell can be rather glitchy, for example, the shapes circling a creature can sometimes stay in place, not moving with the creature and keeping a circle around an empty area. These shapes can also sometimes not keep their ring pattern, and can also partly disappear, causing the ring around a creature to break and lose shape.
It is not known why this spell was not kept in the final game, but it is likely due to balancing issues, or possibly the graphical glitches present. It is believed that had it been put in the game, it would've been a spell belonging to high-level creatures such as the Dark Angel, or possibly endgame opponents.